All of the lead produced in the United States is by pyrometallurgical smelting processes of sulfur-containing ores or feedstocks. The standard methods of obtaining metallic lead from such materials usually include producing concentrates and treating these concentrates in a blast furnace. For example, lead sulfide concentrates containing as much as 75 percent lead are diluted with silica slag to approximately 50 percent lead prior to sintering to produce feed for the blast furnace. During the high temperature sintering process of about 1400.degree. C., substantial and undesirable amounts of lead and sulfur gas emissions occur. The sinter, which consists mainly of lead silicates, is reduced with coke in the lead blast furnace to produce lead bullion, which is an impure lead metal. The blast furnace also contributes significantly to lead and sulfur gas emissions. Fumes collected from the sinter bed and the blast furnace typically will contain 55 percent lead, as well as elements such as arsenic and cadmium, which are also possibly detrimental. A substantial problem with emissions begins in the refinery, where as many as a dozen 250- to 300-ton kettles of lead at temperatures that approach 1,000.degree. C. are in operation at one time. No effective dust or fume collection system exists for such refineries.
Attempts have been made to find a less polluting process for recovery of lead from sulfur-containing materials in view of the recent restrictive environmental standards for lead and sulfur emissions. A prominent process to meet these restrictions is the ferric chloride leach-fused salt electrowinning method. This method avoids lead and sulfur emissions, but the fused salt electrowinning of lead at 500.degree. C. is very demanding of materials used in cell construction and is energy intensive.
The prior art is also aware of other methods for lead recovery from lead-containing materials and recovery of analogous metals from sulfur-containing ores and concentrates. For example, in U.S. Pat. No. 4,149,947 to Stauter et al, metallic lead is obtained from lead-bearing sources, such as lead sulfide ores or concentrates by halogenating the lead source, brine leaching the lead halide to separate the soluble lead halide from solid materials, and recrystallizing the lead halide at a reduced temperature. The lead halide is then converted to lead carbonate by treatment with carbon dioxide, solubilized in hydrofluosilicic acid, and subjected to electrolysis to produce pure lead. In other procedures, U.S. Pat. No. 3,959,436 to Watts describes an oxidative leaching process for leaching metal sulfide minerals. This process comprises leaching a metal sulfide mineral by contacting the mineral with an oxidated leaching solution to recover the metals, which includes the steps of adding an aqueous solution of hydrogen peroxide to the oxidated leaching solution. In U.S. Pat. No. 4,011,146 to Coltrinari et al, metal values are recovered from sulfide ores which contain lead, zinc, and silver sulfides by converting the sulfides to chlorides by chlorination, followed by solubilization with a sodium chloride leach, and subsequent recovery of the metals from their chlorides by a conventional process, which can include fused salt electrolysis. In U.S. Pat. No. 4,440,569 to Weir et al, zinc is recovered from zinc-containing sulfide ores by leaching the sulfide ore under oxidizing conditions in excess sulfuric acid. The undissolved residue which contains the lead and/or silver is then separated and treated to recover the lead values.
There remains a need in the art, however, for a process for the recovery of lead and equivalent metals from sulfur-containing materials which are not subject to the disadvantages of the polluting and energy-intensive processes known to the art.